Known recording materials using a combination of an electron donating colorless dye (hereinafter referred to as color former) and an electron accepting compound (hereinafter referred to as color developer) include pressure-sensitive paper, heat-sensitive paper, light- and pressure-sensitive paper, electric heat-sensitive paper, and the like. The details of these recording materials are described, e.g., in British Patent No. 2,140,449, U.S. Pat. Nos. 4,480,052, 4,436,920, 3,491,112, 3,829,322 and 4,062,866, Japanese patent publication No. 23992/85, (corresponding to U.S. Pat. No. 4,181,328 and British patent No. 1,552,517) and Japanese Patent Application (OPI) Nos. 179836/82, 123556/85, and 123557/85 (the term "OPI" as used herein means an "unexamined published Japanese patent application").
These recording materials are required (1) to provide an image having sufficient color density with sufficient color formation sensitivity, (2) not to fog, (3) to provide an image having sufficient fastness, (4) to form a hue suited for copying machines, (5) to have a high S/N ratio, (6) to provide an image sufficiently resistant to chemicals, (7) to be easy to dissolve in an organic solvent, and the like. Further, compounds used in the preparation of the recording materials must be soluble in organic solvents. However, none of the conventional recording materials has completely fulfilled these requirements.
With the recent diversification of demands for recording systems, extensive studies have been conducted to improve the aforesaid performance properties. Of various recording materials, blue-forming recording materials are particularly sought that satisfy the above requirement (3).
Known compounds which form a blue to bluish purple color include diphenylmethane compounds, triphenylmethane compounds, phthalide compounds, and Leucomethylene Blue type compounds. However, each of the known compounds has its respective disadvantages. For example, 3,3-bis(p-dimethylaminophenyl)-6-dimethylaminophthalide (Crystal Violet Lactone), though developing a deep blue color at high rate, provides an image extremely poor in light-fastness. 3,7-Bis(dimethylamino)-10-benzoylphenothiazine (Benzoyl Leucomethylene Blue) provides a color image exhibiting excellent light-fastness, but has a very low rate of color formation and very inferior color developability if combined with an organic color developer. Further, 3-(4-diethylamino-2-ethoxyphenyl)-3-(1-ethyl-2-methylindol-3-yl)-4 or 7-azaphthalide, known as a phenylindolylazaphthalide compound, has the disadvantages of poor solubility in solvents used for encapsulization and strong self-color-developing properties.
The inventors have investigated each of these color formers and color developers, paying attention to their solubility in oil or water, partition coefficients, pKas, the polarity of substituents, position of substitution, changes in crystallizability and solubility when used in combination, and the like.